1. Field of the Invention
The present invention relates to an image forming apparatus and a control method for the image forming apparatus for correcting a color.
2. Description of the Related Art
In recent years, high picture quality of an output image has been required for a color image forming apparatus employing electrophotography system, inkjet systems, etc., such as a color printer and a color copier. Particularly, density gradation and color stability have a large effect on determining whether image quality is good or poor.
Since density changes occur due to even slight environmental changes particularly in a color image forming apparatus of the electrophotography system, it is necessary to have a unit configured to always keep gradation property of a constant density.
Consequently, in a conventional color image forming apparatus, a sensor which detects a density is provided on a photoconductive drum of the electrophotography system or on a primary transfer belt, and a gradation characteristic of each color of C (cyan), M (magenta), Y (yellow), and K (black) is measured. Additionally, in the color image forming apparatus, mounted is a calibration technology which creates an LUT (Look Up Table) for one-dimensional gradation correction with respect to each color. The LUT is the table indicating output data corresponding to input data separated at a specific interval, and it can represent a nonlinear characteristic which cannot be expressed by an arithmetic expression. The LUT for one-dimensional gradation correction represents each output signal value on the side of an image forming apparatus capable of representing each input signal value of C, M, Y, and K. The image forming apparatus forms an image on a paper using toner corresponding to the output signal value.
In creating the one-dimensional LUT, a chart including a patch of data with different gradation corresponding to each toner of C, M, Y, and K is prepared and output by the image forming apparatus. A value of the chart output by the image forming apparatus is read using the above-described sensor or scanner of the image forming apparatus, or a color measuring instrument (colorimeter) other than the image forming apparatus, etc. A one-dimensional LUT for correction is created independently for C, M, Y, and K by comparing the read value with previously provided target data.
However, since a nonlinear difference occurs in a “color mixture” using a plurality of toner, such as red, green, blue, and gray using CMY, according to the image forming apparatus even though a single color gradation characteristic is corrected by the one-dimensional LUT, it is difficult to assure the color. Consequently, proposed is a technology in which a chart created with the a color mixture in a range where the image forming apparatus can reproduce is output, and the output chart is measured by a scanner or a colorimeter to compare with a target value, and a correction value is created (for example, refer to Japanese Patent Laid-Open No. 2006-165864). For example, proposed is a color mixture calibration technology in which a destination profile that an ICC profile has is focused on, and a color difference of the color mixture is corrected by modifying the destination profile. The ICC profile is data which is used at the time of color conversion defined by an ICC (International Color Consortium). In this technique, first, the chart created with the color mixture is output by the image forming apparatus, and the created chart is measured by a scanner or a colorimeter. A difference is created using the colorimetric result and the target value, and a three-dimensional LUT (destination profile) is updated which converts into a device-dependent color space (CMYK) a device-independent color space (L*a*b*) that the ICC profile has, and whereby it becomes possible to correct a color of the color mixture. The L*a*b* is one of the color spaces independent of the device, L* represents brightness and a*b* represents a hue and saturation.
In addition, in recent years, provided is a system in which a sensor which detects a density and a color is provided at a paper transport section after a fixing process (post fuser) of the electrophotography system instead of a colorimeter connected to a scanner or an outside to read a chart to be output.
According to the above-described technologies, it has been possible to perform color mixture calibration using the post fuser.
Further, also proposed is a technology in which an image forming apparatus itself having the post fuser sensor mounted therein serves as a measuring instrument, and measures printed matter not only of its own apparatus but of other apparatuses (for example, Japanese Patent Laid-Open No. 2003-107833).
In the technology described in Japanese Patent Laid-Open No. 2003-107833, first, the image forming apparatus feeds a measuring image data printed matter of the other apparatus to a printing paper transport path of its own apparatus, and transfers it in a state where electrophotographic process processing is turned off. A density and a color are then measured by the post fuser sensor, and reference data is obtained. After that, the electrophotographic process is turned on, and a same image data is printed from its own apparatus. Similar measurement is performed by the post fuser sensor in this printing process, and a table which corrects a difference with the reference data is generated. The image forming apparatus matches gradation property thereof to gradation property of the other image forming apparatus by application of the above. In addition, it is assumed that calibration in the other apparatus is performed by using a correction table generated as described above.
However, since an ability and a setting for achieving calibration depend on an individual model, calibration has been achieved only in a combination of instruments in which various settings had been previously optimized.
For example, performance of a sensor mounted in each model of the image forming apparatus having a post fuser sensor mounted therein may be different from each other. This is because there exist various types of post fuser sensors in respect of cost or performance depending on variety of a semiconductor manufacturing process technology or an electronic circuit technology which are used for the sensor. For example, it is assumed that models which mount a number of post fuser sensors and models which can mount only a small number of post fuser sensors mixedly exist in the market. In addition, it is assumed that models in which a length of patch data which the sensor can read may be short or must be long depending on the performance of the sensor also mixedly exist in the market.
Further, there is an image forming apparatus which achieves a color mixture calibration function in a 4D-LUT which outputs a new CMYK combination from four input signals of CMYK. However, it is assumed that models having different formats of the number of data entries, a bit depth at the time of data holding of the 4D-LUT, etc. mixedly exist in the market.
The above is individually optimized in the individual model. Accordingly, there is a case where a trouble occurs when instruments are combined so that a chart optimized and printed for a certain model is measured by another model having a post fuser sensor mounted therein. For example, there is a case where mismatch among instruments, such as misalignment of a patch location and a sensor location on the chart, occurs, and thus color mixture calibration is not established.
As described above, there has been a problem that cannot be performed color mixture calibration by cooperation among instruments with respect to an arbitrary combination of the instruments.